The pentahydric alcohol xylitol is the sugar alcohol derived from the reduction of xylose (C.sub.5 H.sub.10 0.sub.5). Xylitol is a naturally occurring, five-carbon sugar alcohol which has the same sweetness and caloric content of sugar (4 kilocalories per gram). Xylitol is found in small amounts in many fruits and vegetables and is produced in the human body during normal metabolism. Xylitol has certain known metabolic, dental and technical characteristics which make it an attractive sugar substitute in various contexts
Xylitol is metabolized independently of insulin, so it can be safely consumed by non-insulin dependent diabetics Further, xylitol has been shown to delay gastric emptying and to possibly suppress food intake which means it may have an important role in weight reducing diets.
Xylitol is also a non-cariogenic, and possibly even a cariostatic substance. In the mouth, sucrose and other carbohydrates are fermented by Streptococcus mutans and other bacteria, generating acid which lowers the pH, demineralizes tooth enamel and leads to dental caries. S. mutans and other acid by-products of fermentation which contribute to tooth decay. Studies have also produced data which suggests that xylitol may even actively suppress the formation of new caries and may even "reverse" existing lesions by inducing remineralization.
From a taste perspective, xylitol does not typically manifest an unpleasant aftertaste like other sugar substitutes and, because of the high energy required to dissolve one gram of xylitol, it produces a pleasant "cooling" effect in the mouth.
Despite xylitol's advantages, the utilization of xylitol on a commercial scale has been limited by its relatively high cost, due to the difficulty of its production on a commercial scale. Xylitol is generally prepared from xylan-containing material, particularly hydrolysates of hemicelluloses. Hemicelluloses are a large group of well characterized polysaccharides found in the primary and secondary cell walls of all land and freshwater plants. Hemicelluloses are made up of sugar residues, among others D-xylose and including D-mannose, D-glucose, D-galactose and L-arabinose.
In prior art methods, xylitol has been prepared from xylan-containing material by hydrolyzing the material to produce a mixture of monosaccharides, including xylose. The xylose is converted to xylitol, generally in the presence of a nickel catalyst such as Raney-nickel. The prior art reveals a number of methods for the production of xylose and/or xylitol from xylan-containing material. Included in such prior art methods are U.S. Pat. No. 3,784,408 (Jaffe et al.), U.S. Pat. No. 4,066,711 (Melaja et al.), U.S. Pat. No. 4,075,405 (Melaja et al.), U.S. Pat. No. 4,008,285 (Melaja et al.) and U.S. Pat. No. 3,586,537 (Steiner et al.).
These prior art methods are, however, complicated multi-step processes which are relatively expensive and inefficient. The prior art recognizes that one of the principal problems in this context is the efficient and complete separation of xylose and/or xylitol from polyols and other by-products of hydrolysis in order to obtain xylitol of sufficient purity. In order to address this fundamental concern, multistep separation techniques, including mechanical filtration and chromatographic separation are generally required. In addition, the art teaches the use of other purification methods, such as the use of acids to precipitate lignins which generally increase the time and expense of xylitol production on a commercial scale.
It is known that certain yeasts possess the enzyme xylose reductase which catalyzes the reduction of D-xylose to xylitol as the first step in D-xylose metabolism. Studies, on an experimental scale, have utilized yeast cells capable of fermenting D-xylose or cell-free extracts containing xylose reductase to produce xylitol from D-xylose rich starting material. Gong, et al., Quantitative Production of Xylitol From D-Xylose By a High Xylitol Producing Yeast Mutant Candida Tropicalis HXP2, Biotechnology Letters, Vol. 3, No. 3, 125-130 (1981); Kitpreechavanich, V. et al.: Conversion of D-Xylose Into Xylitol By Xylose Reductase From Candida Pelliculosa Coupled With the Oxidoreductase System of Methanogen Strain HU, Biotechnology Letters, Vol. 10, 651-656 (1984); McCracken and Gong, Fermentation of Cellulose and Hemicellulose Carbohydrates by Thermotolerant Yeasts, Biotechnology and Bioengineering Symp. No. 12, pp. 91-102 (John Wiley & Sons 1982). Although yeast strains exist which are capable of producing high yields of xylitol from the fermentation of D-xylose, a complete process for producing xylitol from, for example, biomass hemicellulose hydrolysates which contain xylose in addition to hexoses and other impurities on a commercial scale has not been disclosed by the prior art.
The present invention, however, discloses an efficient method of producing relatively high purity xylitol from xylose-containing starting material which utilizes yeast strains capable of converting xylose to xylitol and most hexoses present to ethanol; such fermentation produces a xylitol rich solution from which xylitol can be simply and efficiently recovered without resort to any extensive and expensive separation expedients. Generally, the xylitol can be purified in one step by chromatographic separation and subsequently crystallized. Small amounts of ethanol are easily removed by evaporation or similar expedients, thereby avoiding the need for extensive techniques to separate the xylitol from hexitols and other sugars generated by hydrolysis and conventional hydrogenation and which are present in the xylitol rich solution.